Subsurface Rock Abundances in Light Plains and Ejecta Flows of the Orientale Basin

Smooth deposits with reflectances intermediate of basaltic mares and lunar highlands are widespread on the Moon. Termed light plains, they are mainly formed by ballistic ejecta of impact basins, and recent observations suggest that impact craters smaller than basins are also capable of forming light plains. Light plains are an important component of the lunar shallow crust, and their formation is accompanied by transportation and mixing of materials with different provenances. However, the physical properties of materials in light plains are not well constrained, but they are important to the understanding of the evolution of the lunar shallow crust. This study investigates the subsurface properties of light plains formed by the Orientale basin, and the results are compared to those of cogenetic ejecta flows (EF) formed by the Orientale basin and contemporary basaltic mare nearby. Comparison of radar scattering and thermophysical properties of these units reveals that light plains have higher abundances of centimeter- to meter-sized rock fragments. Based on the spatial distribution of fresh impact craters with and without excavated boulders, subsurface meter-sized boulder distributions are highly heterogeneous across the investigated units, with the greatest heterogeneity observed in light plains emplaced in open topography. The observations suggest that compared to the emplacement of ponded light plains and EF formed by Orientale, emplacement of the light plains in open topography was accompanied by more intense ballistic erosion of the underlying target during re-impact of melt-rich ejecta.